We are studying cellular factors involved in the formation of neuromuscular synapses. Two different but related approaches are used. The first involves growth of nerve and muscle in culture. Embryonic motor neurons are grown in vitro with embryonic or adult (dissociated) skeletal muscle fibers. Functional synapse formation is studied using conventional electrophysiological and electron microscopic techniques. In the second series of experiments, motor nerve sprouting is studied in vivo following partial denervation of a muscle.